The Nation: Down and Dirty; The War in Iraq Turns Ugly. That's What Wars Do.

THIS campaign was begun, like so many others throughout history, with lofty exhortations from battlefield commanders to their troops, urging courage, patience, compassion for the Iraqi people and even chivalry. Within a week it had degenerated into an unexpected ugliness in virtually every populated area where American and British forces have come under fire. Those who believed from intelligence reports and Pentagon war planners that the Iraqi people, and particularly those from the Shiite sections of the southeast, would rise up to greet them as liberators were instead faced with persistent resistance.

Near Basra, as The Financial Times reported, ''soldiers were not being welcomed as liberators but often confronted with hatred.'' In the increasingly messy fights around Nasiriya, Marine units, which earlier were ambushed while responding to what appeared to be a large-scale surrender, had by the end of the week destroyed more than 200 homes.

Visions of cheering throngs welcoming them as liberators have vanished in the wake of a bloody engagement whose full casualties are still unknown. Snippets of news from Nasiriya give us a picture of chaotic guerrilla warfare, replete with hit-and-run ambushes, dead civilians, friendly fire casualties from firefights begun in the dead of night and a puzzling number of marines who are still unaccounted for. And long experience tells us that this sort of combat brings with it a ''downstream'' payback of animosity and revenge.

Other reports corroborate the direction that the war, as well as its aftermath, promises to take: Iraqi militiamen, in civilian clothes, firing weapons and disappearing inside the anonymity of the local populace. So-called civilians riding in buses to move toward contact. Enemy combatants mixing among women and children. Children firing weapons. Families threatened with death if a soldier does not fight. A wounded American soldier commenting, ''If they're dressed as civilians, you don't know who is the enemy anymore.''

These actions, while reprehensible, are nothing more than classic guerrilla warfare, no different in fact or in moral degree from what our troops faced in difficult areas of Vietnam. In the Fifth Marine Regiment area of operations outside Da Nang, we routinely faced enemy soldiers dressed in civilian clothes and even as women. Their normal routes of ingress and egress were through villages, and we fought daily in populated areas. On one occasion a smiling, waving girl -- no more than 7 years old -- lured a squad from my platoon into a vicious North Vietnamese crossfire. And if a Vietcong soldier surrendered, it was essential to remove his family members from their village by nightfall, or they might be killed for the sake of discipline.

The moral and tactical confusion that surrounds this type of warfare is enormous. It is also one reason that the Marine Corps took such heavy casualties in Vietnam, losing five times as many killed as in World War I, three times as many as in Korea and more total casualties than in World War II. Guerrilla resistance has already proved deadly in the Iraq war, and far more effective than the set-piece battles that thus far have taken place closer to Baghdad. A majority of American casualties at this point have been the result of guerrilla actions against Marine and Army forces in and around Nasiriya. As this form of warfare has unfolded, the real surprise is why anyone should have been surprised at all. But people have been, among them many who planned the war, many who are fighting it and a large percentage of the general population.

Why? Partly because of Iraq's poor performance in the 1991 gulf war, which caused many to underestimate Iraqi willingness to fight, while overlooking the distinction between retreating from conquered territory and defending one's native soil. And partly because protection of civilians has become such an important part of military training. But mostly, because the notion of fierce resistance cut against the grain of how this war was justified to the American people.

The strategies of both Iraq and the United States are only partly, some would say secondarily, military. The key strategic prize for American planners has always been the acceptance by Iraq's people of an invasion intended to change their government. If the Iraqis welcomed us, the logic goes, it would be difficult for those on the Arab street, as well as Americans and others who questioned the wisdom of the war, to condemn our presence.

Thus, throughout the buildup to war, the Iraqis were characterized to America -- and to our military -- as so brutally repressed by Saddam Hussein's regime that they would quickly rise up to overthrow him when the Americans arrived. This was clearly the expectation of many American fighting men as they crossed into Iraq. ''Their determination was really a surprise to us all,'' said Brig. Gen. John Kelly of the Marines on Friday. ''What we were really hoping for was just to go through and everyone would wave flags and all that.''

On the other side, the Iraqi regime has used both its ancient history and American support of Israel in appealing to the nationalism of its people to resist an invasion by an outside power. It is as yet unclear which argument is succeeding, although early indications are that the American invasion has stirred up enormous animosity.

The initial bombing campaign was political, aimed at Iraqi leaders. The current effort appears to be increasingly strategic, designed to damage the Iraqi military's better units. After that, the next step is likely to be a series of conventional engagements matching American armored and infantry forces against Iraq's Republican Guard. The United States hopes to force Iraq into fixed-position warfare or even to draw them into a wild attack, where American technological superiority and air power might destroy Iraq's best fighting force.

But Iraq's leaders have reviewed their mistakes in the first gulf war and have also studied the American efforts in Somalia and Kosovo. They will most likely try to draw American units into closer quarters, forcing them to fight even armored battles in heavily populated areas nearer to Baghdad. This kind of fighting would be designed to drive up American casualties beyond the point of acceptability at home, and also to harden Iraqi resolve against the invaders.

If American forces are successful in these engagements, the war may be over sooner rather than later. But if these battles stagnate, guerrilla warfare could well become pandemic, not only in Baghdad but also across Iraq. And even considering the strong likelihood of an allied victory, it is hard to imagine an end point without an extremely difficult period of occupation.

In fact, what will be called an occupation may well end up looking like the images we have seen in places like Nasiriya. Do Iraqis hate Saddam Hussein's regime more deeply than they dislike the Americans who are invading their country? That question will still be with this administration, and the military forces inside Iraq, when the occupation begins, whether the war lasts a few more days or several more months.

Or worse, the early stages of an occupation could see acts of retribution against members of Saddam Hussein's regime, then quickly turn into yet another round of guerrilla warfare against American forces. This point was made chillingly clear a few days ago by the leader of Iraq's major Shiite opposition group, who, according to Reuters, promised armed resistance if the United States remains in Iraq after Saddam Hussein is overthrown.

Welcome to hell. Many of us lived it in another era. And don't expect it to get any better for a while.